The background description provided herein is for the purpose of generally presenting the context of the disclosure. Work of the presently named inventors, to the extent it may be described in this background section, as well as aspects of the description that may not otherwise qualify as prior art at the time of filing, are neither expressly nor impliedly admitted as prior art against the present technology.
Catalysts effective at removing NOx from exhaust emissions are desirable in order to protect the environment and to comport with regulations directed to that purpose. It is preferable that such catalysts convert NOx to inert nitrogen gas, instead of converting NOx to other nitrogen-containing compounds. Catalysts that are effective at low temperature may have additional utility for vehicles.
Increasingly stringent NOx emission and fuel economy requirements for vehicles and automobile engines will require catalytic NOx abatement technologies that are effective under lean-burn conditions. Direct NOx decomposition to N2 and O2 is an attractive alternative to NOx traps and selective catalytic reduction (SCR) for this application, as NOx traps and SCR processes are highly dependent on reductants (such as unburned hydrocarbons or CO) to mitigate NOx. The development of an effective catalyst for direct NOx decomposition would eliminate the use of reducing agents, simplifying the NOx removal process, and therefore decreasing the fuel efficiency cost of NOx abatement.
However, most catalysts active for direct NOx decomposition are only efficient at high temperatures (i.e., greater than about 600° C.), which severely limits a practical application for a vehicle exhaust gas stream. The most well-known low temperature (i.e., less than about 500° C.) direct NOx decomposition catalysts include Cu-ZSM5, K/Co3O4, Na/Co3O4, CuO, and Ag/Co3O4. However, low temperature activity and selectivity to N2 for all of these catalysts is not sufficient for practical application, and more advancements are needed. Advancements in direct NOx decomposition catalysis based on structure activity relationships are lacking, and methodology to improve the performance of specific catalyst systems is needed.
Accordingly, it would be desirable to provide a catalyst for the removal of NOx from exhaust gas, that is effective at low temperature and that has high N2 product specificity.